1. Field of the Invention
The invention relates to a clock generator circuit and a method thereof, and more particularly to a clock generator circuit using phase modulation technology and a method thereof.
2. Description of the Related Art
Spread spectrum can reduce energy having a converged frequency and thus decrease the electromagnetic interference on other electrical devices. FIG. 1 is a schematic block diagram showing a spread spectrum circuit using a phase-locked loop (PLL). The PLL includes a phase frequency detector (PFD) 11, a charge pump 12, a loop filter 13, a voltage controlled oscillator (VCO) 14, and a divider 15. The spread spectrum circuit in the conventional PLL is configured such that a modulation signal is applied to a connection between the charge pump 12 and the loop filter 13 in order to change the output voltage of the loop filter 13 (i.e., the control voltage of the VCO 14) and thus the output clock frequency of the VCO 14, thereby achieving the object of spread spectrum.
When the modulation signal is not applied to the PLL, the VCO 14 is configured to generate an output clock having the frequency which is N times that of a reference clock. The output clock generated by the VCO 14 is divided by the divider 15, and then compared to the reference clock in the PFD 11. The comparison result determines whether the current source in the charge pump 12 absorbs or outputs current. Thus, the charges flow into or out of the capacitor of the loop filter 13, thereby influencing the output voltage of the loop filter 13, wherein the number of charges is directly proportional to the phase difference of the comparison result. Because the loop filter 13 is electrically coupled to the VCO 14, the control voltage of the VCO 14 and thus the output clock frequency thereof change as the output voltage of the loop filter 13 changes. Thus, the frequency difference between the output clock frequency and the reference clock is reduced. When the output clock approximates N times of the reference clock, the feedback operation of the PLL enables the VCO 14 to lock the reference clock. Once the VCO 14 locks the reference clock, the output clock frequency is substantially the same as N times of the reference clock frequency except for some phase differences.
Consequently, when the modulation signal is not applied, the output voltage of the loop filter 13 is a constant value while the output clock of the VCO 14 has a constant frequency. However, in order to achieve the object of spread spectrum, the modulation signal is applied to the connection between the charge pump 12 and the loop filter 13. The modulation signal is for interfering with the input voltage of the loop filter 13 in order to change the output voltages of the charge pump 12 and the loop filter 13, and thus the control voltage of the VCO 14. The waveforms at points A and B of FIG. 1 are shown in FIGS. 2A and 2B, respectively, wherein point A is an input terminal of the VCO 14 and point B is an output terminal of the VCO 14. As shown in the drawings, because the modulation signal is added, the voltage value at the input terminal (point A) of the VCO 14 is changing such that its output clock frequency is also changing therewith, and the object of spread spectrum may be achieved.
However, the conventional spread spectrum method performed by applying the modulation signal to the PLL has the following drawbacks.
1. Because the loop bandwidth of the PLL is smaller than the frequency of the modulation signal, the area of the capacitor in the loop filter 13 has to be enlarged, and thus the volume of the circuit device is also enlarged.
2. The conventional method can only perform the center spread spectrum but cannot perform the down spread spectrum. In general, the condition of the up spread spectrum may have the problem that the speeds of other devices in the system are not sufficiently high, but the condition of the down spread spectrum is usually free from the problem of insufficient speed.
3. Because the loop bandwidth of the PLL is too small, the waveform of the output clock of the VCO 14 turns into a quasi-sine wave, as shown in FIGS. 3A and 3B, wherein its ideal frequency spectrum and actual frequency spectrum are illustrated in FIGS. 4A and 4B. As shown in the drawings, because the energy is converged in f1 and f2, the curve of the actual frequency spectrum cannot be a smoother curve, and the associated requirement and specification for the electromagnetic interference also cannot be met.